Part of the in-line process of preparing film support for emulsion coating in a film sensitizing machine is an electrostatic charge control operation. In film coating operations, it is necessary to control both free charge and polar charge, two different manifestations of charge imbalance. Free charge is when a bulk section of the web has more charge of one polarity than the other polarity. Polar charge is when a section of the web has the exact same number of charges of each polarity, but one surface has more of the positive charges and the other has more negative charges.
Two methods of controlling free charge on a moving web include; a brush discharger composed of fine wires which is only effective when the charge density is high; and an AC ionizer which leaves a 60 Hz distribution of charge on the web that can cause coating non-uniformities. Thus, these prior art methods are not totally effective at removing free charge.
FIG. 1 shows a third prior art method of controlling free charge on a moving web 12. The free charge is controlled by a pair of DC ionizers 13 with grounded screens, one a negative ionizer, the other a positive ionizer and a feedback control mechanism to control the corona wire voltage on the positive ionizer. The feedback mechanism includes a sensor 18 which measures the field on the web several feet after the DC ionizers, a controller (not shown), and a controllable high voltage power supply (not shown) to supply the voltage to the positive ionizer corona wire. The negative ionizer is run at a fixed corona wire voltage. Since a positive ionizer has a greater ion output than an identical negative ionizer run at the same voltage (but opposite polarity) controlling only the positive ionizer voltage is a sufficient means to control the net effect of the pair of ionizers.
The dual DC ionizer scheme is more effective at lower charge densities than the brush discharger and it does not impart a frequency to the web as does the AC ionizer scheme. The dual DC ionizer scheme as shown in FIG. 1 does, however, have two notable disadvantages. It is not very effective on moderately low charge densities and its ability to control the charge level tends to degrade as the back side charge density of the incoming web is reduced. As a result, on some supports the electric field on the web following a discharger can wander from approximately -2000 to +2000 volts per inch or so. The lack of control raises the variability of the process, in that operating at higher electric field strengths may be enough to attract airborne particulate matter to the web, and these can become incorporated into the film during coating. In addition, the electric field on the web increases the severity of coating imperfections caused by dirt and debris.
The present invention solves the problem of prior art charge control devices. It controls charge on the web, even at moderate levels, and it prevents wandering of the charge, and it imparts no charge frequency on the web.